PROJECT SUMMARY Multiple Sclerosis (MS) is an autoimmune disease of the central nervous system (CNS) that constitutes the leading cause of neurologic disability in young adults. Astrocytes play important roles in MS and its model, experimental autoimmune encephalomyelitis (EAE), but the regulation of astrocyte activity is poorly understood. The study of astrocyte regulation is likely to identify mechanisms of pathogenesis and therapeutic targets in MS, particularly for its secondary progressive phase for which no efficacious therapies are available. We identified a pathway controlled by the ligand-activated transcription factor aryl hydrocarbon receptor (AhR) that regulates astrocyte function during inflammation. Briefly, AhR in astrocytes promotes the expression of Suppressor Of Cytokine Signaling 2 (SOCS2), an inhibitor of NF-?B activation. AhR deletion in astrocytes up-regulates NF-?B activation and worsens EAE. These data suggest that AhR/SOCS2 signaling suppresses astrocyte pathogenic activities in EAE by limiting NF-?B activation. In support of this interpretation, nasal administration of the AhR agonist ITE suppresses EAE through a mechanism mediated by AhR in astrocytes. Moreover, agonists derived from dietary tryptophan (Trp) and the gut flora ameliorate EAE via AhR in astrocytes. These AhR agonists are decreased in MS patients, suggesting that deficient AhR activation contributes to disease pathogenesis. Collectively, these findings identify a novel AhR/SOCS2 pathway that integrates endogenous and environmental signals to control astrocyte function. We hypothesize that AhR in astrocytes limits CNS inflammation and is a potential therapeutic target for MS. Our specific aims are: SPECIFIC AIM 1: DOES AHR SUPPRESS ASTROCYTE-DRIVEN NEUROTOXICITY DURING EAE? We propose to: 1) Determine the effects of AhR in astrocytes on neurotoxicity during EAE, 2) Establish the effects of SOCS2 on NF-?B activation and astrocyte neurotoxicity, 3) Determine whether AhR limits NF-?B- driven astrocyte neurotoxicity via SOCS2. SPECIFIC AIM 2: HOW DOES AHR CONTROL THE TRANSCRIPTIONAL PROGRAM OF ASTROCYTES? We propose to: 1) Determine the role of AhR in the control of the transcriptional response of astrocytes during EAE, 2) Define the contribution of SOCS2 to the transcriptional effects of AhR in astrocytes, 3) Establish the heterogeneity of astrocyte responses and their control by AhR during EAE by single cell RNA-Seq. SPECIFIC AIM 3: IS AHR A THERAPEUTIC TARGET TO MODULATE ASTROCYTE FUNCTION? We propose to: 1) Evaluate the effects on astrocytes and CNS inflammation of AhR activation by ITE administered nasally, 2) Define the role of AhR/SOCS2 in astrocytes on the chronic progressive NOD EAE, which models important aspects of SPMS, 3) Evaluate the therapeutic value of nasal ITE in NOD EAE. IN SUMMARY, this project uses unique experimental systems to study the role of a novel AhR/SOCS2 pathway in astrocytes on CNS inflammation and its potential as a therapeutic target.